The overarching goals of this proposal are to further our understanding of the effect of potent antiretroviral therapy (ART) on Human Immunodeficiency Virus Type 2 (HIV-2) infection. HIV-2 is generally less pathogenic than HIV-1. Compared to HIV-1, HIV-2 infection is characterized by a much longer asymptomatic stage, lower plasma viral loads, slower decline in CD4+ T cell count, decreased mortality rate due to AIDS, lower rates of mother to child transmission, and lower rates of sexual transmission. Nonetheless, a significant proportion of HIV-2 infected individuals eventually progress to AIDS. Antiretrov'iral therapy is becoming increasingly available in resource-limited settings. In Senegal where both HIV-1 and HIV-2 are found, a national program of providing antiretroviral treatment (ART) for those with AIDS, CD4+ T cell counts less than 200/mm3, or CD4+ T cell counts less than 350/mm3 and clinical symptoms has been established. However, the effects of ART on outcome, immunologic response, viral load, dynamics and evolution, HIV-2 drug resistance and fitness in HIV-2 infected individuals are largely unknown. Our longstanding longitudinal cohort of HIV-2 infected subjects from Dakar, Senegal provides a unique opportunity to study the role ART plays in suppressing HIV-2 infection. The Specific Aims of this proposal are: AIM 1: To determine the clinical, virologic and immunologic outcomes associated with antiretroviral therapy (ART) in a longitudinal prospective HIV-2 infected Senegalese cohort. AIM 2: To assess role of drug resistance in viruses that emerge in HIV-2 infected subjects with virologic failure and to determine if there are qualitative and quantitative differences the drug resistant viruses that emerge between HIV-2 and HIV-1.